AgDM newsletter article, February 1999

Making the most of soil nutrient dollars

John Sawyerby John Sawyer, Extension Soil Fertility Specialist, 515.294.1923, jsawyer@iastate.edu

Crop producers may be able to temporarily adjust their fertilizer and limestone use when facing a financial crunch. The key is to address critical needs in order to provide acceptable profitability for the short term, while at the same time minimizing negative effects of managing with less than optimal resources. When finances improve, producers can re-focus on areas previously given lower priority.

But soil testing is one management practice producers can't afford to skip.  Testing is the only way to understand the potential need for fertilization and liming. Test results can then be augmented with field- or area-specific soil productivity records to determine nutrient needs that correlate with reasonable yield expectations.

Liming

Producers should consider adding lime if pH falls below 6.0 for straight grass or grass hay pastures; below 6.4 for corn and soybean; and below 6.8 for alfalfa. Traditional recommendations suggest increasing pH to 6.5 for grass, corn, and soybean and to 6.9 for alfalfa.

When cost is a critical concern, a short-term, prioritized application strategy can be employed:

Because limestone applications correct soil pH for several years, and liming is a long-term management program, pH is corrected for several crops (in a rotation), and therefore costs can be amortized. The long-term nature of liming poses complications when a producer attempts to make short-term decisions. While the above-mentioned strategies can help with lime allocation in the short term, potential problems from low soil pH, as well as similar lime-use questions, will arise when fields bypassed this year are rotated in future years.

Nitrogen (N)

For profitable corn production, it's most important to determine the amount of N required, then allocate financial resources to ensure those applications. Accounting for and using N from other sources--such as rotation after alfalfa and soybean, manure, and other byproducts and fertilizers--can, with proper management, significantly offset the need for purchased N, keeping costs down.

Crop price and N cost must be balanced to determine the most economical N rate. With a corn price range from $3.00 to $1.50/bu, reduction in optimum N rate is not large unless N costs are high; thus the anticipated price for corn must be carefully considered. Here, the late-spring soil nitrate test can help with field-specific decisions, particularly following manure applications, because it can assure that N expected from manure is available to the crop.

Risk of N loss is an important concern when a producer refines N rates to save money. Making spring preplant applications close to planting date, as well as sidedressing, can reduce that risk.  Late fall application can be comparable if weather and soil conditions are favorable, but N loss may be greater then due to longer-term environmental exposure.  Limit fall applications to situations with low N loss potential, and waiting until soils have cooled enough to slow nitrification (temperature at the 4-inch depth of 50 degrees F and cooling).

Phosphorus and potassium (P and K)

The highest application priority for P and K is where soil tests indicate greatest potential return on the investment (that is, soils testing very low and low). For test results at these levels, inadequate fertilizer applications will reduce yield and profitability. Available manure also should be targeted to these areas.

Applying P and K to soils testing optimum may increase yields, but the return is not as great as for lower soil test results. Maintaining optimal P and K levels is profitable in the long term, but when finances are tight, such applications could be reduced.

On soils testing above optimum, P and K applications are optional, but if they are skipped, increased fertilization will be needed later. Soils testing very high can have P and K withheld for several years with no adverse yield effects. Again, regular testing is essential to monitor changes in fertility.

Starter should be applied to corn if it has produced response in the past. If reducing P and K rates is absolutely necessary, two by two starter or banding can enhance efficiency. Most manure contains significant crop-available P and K and in many instances can supply the needs of one or more crops.

Secondary and micronutrients

Secondary or micronutrients should be applied only when deficiencies have been confirmed. These situations, which generally involve special soil and climatic conditions, should be addressed with target applications only. Most Iowa soils are adequate in such nutrients and the likelihood of yield enhancement through their application is low compared to the frequency of yield increases observed for N, P, K, and liming.

 

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